Chemically amplified resist composition and pattern forming process

ABSTRACT

A resist composition is provided comprising (A) a carboxylic acid sulfonium salt whose anion moiety has a bulky structure of arenecarboxylate in which secondary or tertiary carbon atoms bond at both ortho-positions relative to the carbon atom in bond with carboxylate, as an acid diffusion regulator and (B) a polymer which is decomposed under the action of acid to increase its solubility in alkaline developer. When processed by EB or EUV lithography, the resist composition exhibits a very high resolution and forms a pattern with minimal LER.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2014-118577 filed in Japan on Jun. 9, 2014,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a chemically amplified resist compositionwhich is sensitive to high-energy radiation such as UV, DUV, EB, EUV,X-ray, gamma-ray or synchrotron radiation, and useful in processing ofsemiconductors and photomask blanks, and more particularly, to achemically amplified resist composition which is amenable to the step ofexposure to a beam of high-energy radiation such as EB or DUV, and apattern forming process using the resist composition.

BACKGROUND ART

To meet the recent demand for higher integration in integrated circuits,pattern formation to a finer feature size is required. Acid-catalyzedchemically amplified resist compositions are most often used in formingresist patterns with a feature size of 0.2 μm or less. High-energyradiation such as UV, deep-UV or electron beam (EB) is used as the lightsource for exposure of these resist compositions. In particular, whileEB lithography is utilized as the ultra-fine microfabrication technique,it is also indispensable in processing a photomask blank to form aphotomask for use in semiconductor device fabrication.

Polymers comprising a major proportion of aromatic structure having anacidic side chain, for example, polyhydroxystyrene have been widely usedin resist materials for the KrF excimer laser lithography. Thesepolymers are not used in resist materials for the ArF excimer laserlithography since they exhibit strong absorption at a wavelength ofaround 200 nm. These polymers, however, are expected to form usefulresist materials for the EB and EUV lithography for forming patterns offiner size than the processing limit of ArF excimer laser because theyoffer high etching resistance.

Often used as the base polymer in positive resist compositions for EBand EUV lithography is a polymer having an acidic functional group onphenol side chain masked with an acid labile protective group. Uponexposure to high-energy radiation, the acid labile protective group isdeprotected by the catalysis of an acid generated from a photoacidgenerator so that the polymer may turn soluble in alkaline developer.Typical of the acid labile protective group are tertiary alkyl,tert-butoxycarbonyl, and acetal groups. On use of protective groupsrequiring a relatively low level of activation energy for deprotectionsuch as acetal groups, a resist film having a high sensitivity isadvantageously obtainable. However, if the diffusion of generated acidis not fully controlled, deprotection reaction can occur even in theunexposed regions of the resist film, giving rise to problems likedegradation of line edge roughness (LER) and a lowering of in-planeuniformity of pattern line width (CDU).

Improvements were made in the control of resist sensitivity and patternprofile by properly selecting and combining components used in resistcompositions and adjusting processing conditions. One outstandingproblem is the diffusion of acid that has a material impact on theresolution of a chemically amplified resist composition. Many studiesare made on the acid diffusion problem because sensitivity andresolution are largely affected thereby.

An acid diffusion regulator is, in fact, essential for controlling aciddiffusion and improving the performance of a resist composition. Studieshave been made on the acid diffusion regulator while amines and weakacid onium salts have been generally used. The weak acid onium salts areexemplified in several patent documents. JP 3955384 describes that theaddition of triphenylsulfonium acetate ensures to form a satisfactoryresist pattern without T-top profile, a difference in line width betweenisolated and grouped patterns, and standing waves. JP-A H11-327143reports improvements in sensitivity, resolution and exposure margin bythe addition of sulfonic acid ammonium salts or carboxylic acid ammoniumsalts. Also, JP 4231622 describes that a resist composition for KrF orEB lithography comprising a PAG capable of generating a fluorinatedcarboxylic acid is improved in resolution and process latitude such asexposure margin and depth of focus. Further, JP 4116340 describes that aresist composition for F₂ laser lithography comprising a PAG capable ofgenerating a fluorinated carboxylic acid is improved in line edgeroughness (LER) and solves the footing problem. While these four patentdocuments refer to the KrF, EB and F₂ lithography, JP 4226803 describesa positive photosensitive composition for ArF excimer laser lithographycomprising a carboxylic acid onium salt. These systems are based on themechanism that a salt exchange occurs between a weak acid onium salt anda strong acid (sulfonic acid) generated by another PAG upon exposure, toform a weak acid and a strong acid onium salt. That is, the strong acid(sulfonic acid) having high acidity is replaced by a weak acid(carboxylic acid), thereby suppressing acid-catalyzed decompositionreaction of acid labile group and reducing or controlling the distanceof acid diffusion. The onium salt apparently functions as an aciddiffusion regulator.

However, when a resist composition comprising the foregoing carboxylicacid onium salt or fluorocarboxylic acid onium salt is used inpatterning, a problem of LER arises. It would be desirable to have anacid diffusion regulator capable of minimizing LER.

CITATION LIST

-   Patent Document 1: JP 3955384 (U.S. Pat. No. 6,479,210)-   Patent Document 2: JP-A H11-327143-   Patent Document 3: JP 4231622 (U.S. Pat. No. 6,485,883)-   Patent Document 4: JP 4116340 (U.S. Pat. No. 7,214,467)-   Patent Document 5: JP 4226803 (U.S. Pat. No. 6,492,091)

DISCLOSURE OF INVENTION

An object of the invention is to provide a chemically amplified resistcomposition which is processed by lithography to form a resist patternwith minimal LER, and a pattern forming process using the resistcomposition.

The inventors have found that a resist composition comprising asulfonium salt having the general formula (1) defined below can beprocessed by lithography to form a resist pattern with minimal LER.

In one aspect, the invention provides a chemically amplified resistcomposition comprising (A) a sulfonium salt having the general formula(1) and (B) a polymer comprising recurring units having the generalformula (U-1), which is decomposed under the action of acid to increaseits solubility in alkaline developer.

Herein R¹¹ and R²² are each independently a branched or cyclic C₃-C₂₀monovalent hydrocarbon group which may be substituted with or separatedby a heteroatom, R³³ and R⁰¹ are each independently hydrogen, or astraight C₁-C₂₀ or branched or cyclic C₃-C₂₀ monovalent hydrocarbongroup which may be substituted with or separated by a heteroatom, k isan integer of 0 to 4, or R¹¹, R²², R³³ and R⁰¹ may bond together to forma ring with the carbon atoms to which they are attached and the carbonatom or atoms therebetween, m is 0 or 1, R¹⁰¹, R¹⁰² and R¹⁰³ are eachindependently a straight C₁-C₂₀ or branched or cyclic C₃-C₂₀ monovalenthydrocarbon group which may be substituted with or separated by aheteroatom, or any two or more of R¹⁰¹, R¹⁰² and R¹⁰³ may bond togetherto form a ring with the sulfur atom.

Herein q is 0 or 1, r is an integer of 0 to 2, R¹ is hydrogen, fluorine,methyl or trifluoromethyl, R² is each independently hydrogen or C₁-C₆alkyl, B¹ is a single bond or C₁-C₁₀ alkylene which may contain an etherbond, a is an integer satisfying a≦5+2r−b, and b is an integer of 1 to3.

In a preferred embodiment, component (A) is selected from the followingsulfonium salts (A-30) to (A-44).

In a preferred embodiment, the resist composition may further comprisean acid generator capable of generating at least one acid selected fromsulfonic acids, imidic acids, and methide acids, upon exposure tohigh-energy radiation.

In a preferred embodiment, the polymer further comprises recurring unitshaving the general formula (U-2).

Herein s is 0 or 1, t is an integer of 0 to 2, R¹, R², and B¹ are asdefined above, c is an integer satisfying c≦5+2t−e, d is 0 or 1, e is aninteger of 1 to 3, X is an acid labile group when e is 1 or X ishydrogen or an acid labile group when e is 2 or 3, at least one X beingan acid labile group.

In a preferred embodiment, the polymer further comprises recurring unitshaving the general formula (U-3) and/or (U-4).

Herein f is an integer of 0 to 6, R³ is each independently hydrogen, anoptionally halo-substituted C₁-C₆ alkyl or primary or secondary alkoxygroup, or an optionally halo-substituted C₁-C₇ alkylcarbonyloxy group, gis an integer of 0 to 4, and R⁴ is each independently hydrogen, anoptionally halo-substituted C₁-C₆ alkyl or primary or secondary alkoxygroup, or an optionally halo-substituted C₁-C₇ alkylcarbonyloxy group.

In a preferred embodiment, the resist composition further comprises abasic compound.

Typically the resist composition is subject to ArF, KrF, EB, EUV orX-ray lithography.

In another aspect, the invention provides a pattern forming processcomprising the steps of applying the chemically amplified resistcomposition defined above onto a processable substrate to form a resistfilm, exposing patternwise the resist film to high-energy radiation, anddeveloping the exposed resist film in an alkaline developer to form aresist pattern.

Preferably, the processable substrate has the outermost surface of achromium-containing material. Typically, the processable substrate is aphotomask blank.

Advantageous Effects of Invention

When the inventive resist composition comprising the sulfonium salt isprocessed by a micropatterning process, typically EB or EUV lithography,a resist pattern having a very high resolution and minimal LER isformed.

DESCRIPTION OF EMBODIMENTS

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. The notation(Cn-Cm) means a group containing from n to m carbon atoms per group.

The abbreviations and acronyms have the following meaning.

EB: electron beam

DUV: deep ultraviolet

EUV: extreme ultraviolet

Mw: weight average molecular weight

Mn: number average molecular weight

Mw/Mn: molecular weight distribution or dispersity

GPC: gel permeation chromatography

PEB: post-exposure bake

PAG: photoacid generator

LER: line edge roughness

Briefly stated, one embodiment of the invention is a chemicallyamplified resist composition comprising a sulfonium salt having thegeneral formula (1).

Herein R¹¹ and R²² are each independently a branched or cyclic C₃-C₂₀monovalent hydrocarbon group which may be substituted with or separatedby a heteroatom. R³³ and R⁰¹ are each independently hydrogen, or astraight C₁-C₂₀ or branched or cyclic C₃-C₂₀ monovalent hydrocarbongroup which may be substituted with or separated by a heteroatom, k isan integer of 0 to 4. R¹¹, R²², R³³ and R⁰¹ may bond together to form aring with the carbon atoms to which they are attached and if any, thecarbon atom or atoms therebetween, m is 0 or 1. R¹⁰¹, R¹⁰² and R¹⁰³ areeach independently a straight C₁-C₂₀ or branched or cyclic C₃-C₂₀monovalent hydrocarbon group which may be substituted with or separatedby a heteroatom, or any two or more of R¹⁰¹, R¹⁰² and R¹⁰³ may bondtogether to form a ring with the sulfur atom in the formula.

Suitable branched or cyclic monovalent hydrocarbon groups of R¹¹ and R²²include isopropyl, sec-butyl, tert-butyl, tert-amyl, cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, cyclooctyl, cyclopentylmethyl,cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl,cyclohexylbutyl, norbornyl, adamantyl, and 4-tetrahydropyranyl.

R³³ and R⁰¹ are each independently hydrogen, or a straight C₁-C₂₀ orbranched or cyclic C₃-C₂₀ monovalent hydrocarbon group which may besubstituted with or separated by a heteroatom. Suitable monovalenthydrocarbon groups include methyl, ethyl, propyl, isopropyl, n-butyl,sec-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl, n-octyl, n-nonyl,n-decyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, cyclopentylmethyl,cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl,cyclohexylbutyl, norbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, adamantyl,and adamantylmethyl.

In the hydrocarbon groups of R¹¹, R²², R³³ and R⁰¹, a hydrogen atom (oratoms) may be substituted by a heteroatom such as oxygen, sulfur,nitrogen or halogen, or a heteroatom such as oxygen, sulfur or nitrogenmay intervene. As a result, a hydroxyl, cyano, carbonyl, ether bond,ester bond, sulfonic acid ester bond, carbonate bond, lactone ring,sultone ring, carboxylic anhydride or haloalkyl group may form orintervene.

In formula (1), k indicative of the number of R⁰¹ is an integer in therange: 0≦k≦2 in case of m=0 and in the range: 0≦k≦4 in case of m=1. R¹¹,R²², R³³ and R⁰¹ may bond together to form a ring with the carbon atomsto which they are attached and if any, the carbon atom or atomstherebetween. Suitable cyclic substituent groups thus formed includecyclopentyl, cyclohexyl, and norbornyl, in which a hydrogen atom (oratoms) may be substituted by a heteroatom such as oxygen, sulfur,nitrogen or halogen, or a heteroatom such as oxygen, sulfur or nitrogenmay intervene. As a result, a hydroxyl, cyano, carbonyl, ether bond,ester bond, sulfonic acid ester bond, carbonate bond, lactone ring,sultone ring, carboxylic anhydride or haloalkyl group may form orintervene. In formula (1), the value of m is 0 or 1, with m=0 beingpreferred.

Examples of the preferred structure of the anion moiety in the sulfoniumsalt having formula (1) are shown below, but the sulfonium salt is notlimited thereto.

Of these, structures (A-30) to (A-44) are especially preferred as theanion moiety in the sulfonium salt. A sulfonium salt having an anion ofsuch structure is best suited for use in resist compositions, because itis highly lipophilic, despite a carboxylic acid salt, by virtue of abranched or alicyclic hydrocarbon on an aromatic ring, and because thecarboxylic acid generated therefrom is restricted in diffusion by thesurrounding steric hindrance.

In formula (1), R¹⁰¹, R¹⁰² and R¹⁰³ are each independently a straightC₁-C₂₀ or branched or cyclic C₃-C₂₀ monovalent hydrocarbon group whichmay be substituted with or separated by a heteroatom, or any two or moreof R¹⁰¹, R¹⁰² and R¹⁰³ may bond together to form a ring with the sulfuratom in the formula. Suitable monovalent hydrocarbon groups includealkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl,tert-butyl, cyclopropyl, cyclopentyl, cyclohexyl, cyclopropylmethyl,4-methylcyclohexyl, cyclohexylmethyl, norbornyl, and adamantyl; alkenylgroups such as vinyl, allyl, propenyl, butenyl, hexenyl, andcyclohexenyl; aryl groups such as phenyl, naphthyl and thienyl; andaralkyl groups such as benzyl, 1-phenylethyl, and 2-phenylethyl, withthe aryl groups being preferred. In these hydrocarbon groups, a hydrogenatom (or atoms) may be substituted by a heteroatom such as oxygen,sulfur, nitrogen or halogen, or a heteroatom such as oxygen, sulfur ornitrogen may intervene. As a result, a hydroxyl, cyano, carbonyl, etherbond, ester bond, sulfonic acid ester bond, carbonate bond, lactonering, sultone ring, carboxylic anhydride or haloalkyl group may form orintervene.

Alternatively, any two or more of R¹⁰¹, R¹⁰² and R¹⁰³ may bond togetherto form a ring with the sulfur atom in the formula. Suitable cyclicstructures are shown below.

Herein R⁵ is a group as defined and exemplified above for R¹⁰¹, R¹⁰²,and R¹⁰³.

Examples of the preferred structure of the cation moiety in thesulfonium salt having formula (1) are shown below, but the invention isnot limited thereto.

Exemplary structures for the sulfonium salt include arbitrarycombinations of anions with cations, both as exemplified above.

In the resist composition, the sulfonium salt having formula (1)generates a corresponding arylcarboxylic acid upon exposure. Thegenerated carboxylic acid has a pKa value of about 3.0 to 4.0, that is,relatively low acidity. Thus the generated carboxylic acid isinsufficient to incur deprotection reaction by breaking the acetal bondprotecting a phenolic hydroxyl group on the polymer in the resistcomposition.

Nevertheless, the sulfonium salt having formula (1) can serve as an aciddiffusion regulator. When the sulfonium salt having formula (1) is usedas an acid diffusion regulator along with a PAG capable of generating anacid having high acidity, typically an alkane- or arenesulfonic acid, aresist pattern having low roughness is formed as compared with the useof conventional amine type acid diffusion regulators.

This may be accounted for by the following mechanism. When the sulfoniumsalt having formula (1) is co-present with a strong acid-generatingonium salt capable of generating an acid having relatively high acidity,the salts generate corresponding carboxylic acid and strong acid in aregion exposed to radiation. In a less dose region, most of the oniumsalt remains undecomposed. The strong acid functions as a catalyst totrigger deprotection reaction to the base resin while the carboxylicacid generated by the sulfonium salt having formula (1) induces littledeprotection reaction. The strong acid undergoes ion exchange with theremaining carboxylic acid sulfonium salt, whereupon an onium salt ofstrong acid is formed, and carboxylic acid is released instead.Differently stated, via ion exchange, the strong acid is neutralizedwith the inventive sulfonium salt.

In general, the onium salt type quenchers tend to form a resist patternwith a lower LER than the amine compound quenchers. The reason is asfollows. Salt exchange between strong acid and carboxylic acid sulfoniumsalt is repeated infinitely. The site where strong acid is generated atthe end of exposure is different from the initial site where the strongacid-generating onium salt is present. Since the cycle of photo-acidgeneration and salt exchange is repeated over and over, the acidgeneration point is averaged, which leads to a resist pattern withreduced LER after development.

Some examples of weak acid onium salt serving as the acid diffusionregulator are known in the art. For example, Patent Document 1 disclosestriphenylsulfonium acetates, and Patent Document 2 describes sulfonicacid ammonium salts and carboxylic acid ammonium salts. These salts,however, fail to exert satisfactory performance, especially LERreduction, when applied to the advanced semiconductor materials underminiaturization requirement.

Patent Document 3 describes a resist composition for KrF and EBlithography, comprising a base resin having an acid labile group and aPAG capable of generating a fluorinated carboxylic acid. Since thefluorinated carboxylic acid has a higher acidity than thenon-fluorinated carboxylic acid, it may react with the acid labile groupon the base resin. It does not function as the acid diffusion regulator.

By contrast, the carboxylic acid sulfonium salt of the invention ischaracterized in that the anion moiety has a bulky structure ofarenecarboxylate in which secondary or tertiary carbon atoms bond atboth ortho-positions relative to the carbon atom in bond withcarboxylate.

In general, the carboxylic acid onium salt is less soluble in resistsolvents and tends to remain as foreign particles, causing defects. Theonium salt of the invention is highly lipophilic and fully compatiblewith resist components, with a least possibility to cause defects asforeign particles.

Although the weak acid onium salt is generally insufficient to cleavethe acid labile group on the base resin, there is still a possibilitythat it functions as a photoacid generator in a fully exposed region.Since the onium salt of the invention has steric hindrance aroundcarboxylate as pointed out above, the carboxylic acid generatedtherefrom does not react with the acid labile group on the base resinand purely functions only as the acid diffusion regulator. As a result,the lithography performance, especially LER is improved over theconventional weak acid onium salts.

The resist composition of the invention is characterized by comprisingthe sulfonium salt of formula (1) and a polymer comprising recurringunits of the general formula (U-1) as base resin.

Herein q is 0 or 1, r is an integer of 0 to 2, R¹ is hydrogen, fluorine,methyl or trifluoromethyl, R² is each independently hydrogen or C₁-C₆alkyl, B¹ is a single bond or C₁-C₁₀ alkylene which may contain an etherbond, a is an integer satisfying a≦5+2r−b, and b is an integer of 1 to3.

Examples of the recurring unit having formula (U-1) are shown below.

Of the illustrated units of formula (U-1), units (U-1)-1 to (U-1)-6 arepreferred, with unit (U-1)-1 being most preferred.

While the recurring units of formula (U-1) are essential, the polymermay comprise other recurring units, preferably recurring units of thestructure having the general formula (U-2).

Herein s is 0 or 1, t is an integer of 0 to 2, R¹, R², and B¹ are asdefined above, c is an integer satisfying c≦5+2t−e, d is 0 or 1, e is aninteger of 1 to 3. When e is 1, X is an acid labile group. When e is 2or 3, X is hydrogen or an acid labile group, at least one X being anacid labile group.

The acid labile group is typically selected from tertiary alkyl groupsand acetal groups. Suitable tertiary alkyl groups include tert-butyl,tert-pentyl, 1-ethyl-1-methylpropyl, 1,1-diethylpropyl,1,1,2-trimethylpropyl, 1-adamantyl-1-methylethyl,1-methyl-1-(2-norbornyl)ethyl, 1-methyl-1-(tetrahydrofuran-2-yl)ethyl,1-methyl-1-(7-oxanorbornan-2-yl)ethyl, 1-methylcyclopentyl,1-ethylcyclopentyl, 1-propylcyclopentyl, 1-cyclopentylcyclopentyl,1-cyclohexylcyclopentyl, 1-(2-tetrahydrofuryl)cyclopentyl,1-(7-oxanorbornan-2-yl)cyclopentyl, 1-methylcyclohexyl,1-ethylcyclohexyl, 1-cyclopentylcyclohexyl, 1-cyclohexylcyclohexyl,2-methyl-2-norbornyl, 2-ethyl-2-norbornyl,8-methyl-8-tricyclo[5.2.1.0²⁶]decyl,8-ethyl-8-tricyclo[5.2.1.0^(2,6)]decyl,3-methyl-3-tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodecyl,3-ethyl-3-tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodecyl,2-methyl-2-adamantyl, 2-ethyl-2-adamantyl, 1-methyl-3-oxo-1-cyclohexyl,1-methyl-1-(tetrahydrofuran-2-yl)ethyl, 5-hydroxy-2-methyl-2-adamantyl,and 5-hydroxy-2-ethyl-2-adamantyl.

A typical acetal group is a partial structure having the general formula(U-5):

wherein R⁶ is hydrogen or a straight, branched or cyclic C₁-C₁₀ alkylgroup, and Y is a straight, branched or cyclic C₁-C₃₀ alkyl group.

Examples of the structure having formula (U-5) are shown below.

Herein R⁶ is as defined above.

In formula (U-5), R⁶ is hydrogen or a straight, branched or cyclicC₁-C₁₀ alkyl group. A choice of R⁶ may depend on the designedsensitivity of acid labile group to acid. For example, hydrogen isselected when the acid labile group is designed to ensure relativelyhigh stability and to be decomposed with strong acid. A straight alkylgroup is selected when the acid labile group is designed to haverelatively high reactivity and high sensitivity to pH changes. Althoughthe choice varies with a particular combination of acid generator andbasic compound in the resist composition, R⁶ is preferably a group inwhich the carbon in bond with acetal carbon is secondary, when the acidlabile group is designed to have a relatively large alkyl groupsubstituted at the end and a substantial change of solubility upondecomposition. Examples of R⁶ bonded to acetal carbon via secondarycarbon include isopropyl, sec-butyl, cyclopentyl, and cyclohexyl.

Another choice of acid labile group is to bond (—CH₂COO-tertiary alkyl)to a phenolic hydroxyl group. The tertiary alkyl group used herein maybe the same tertiary alkyl group as used for the protection of phenolichydroxyl group.

The polymer may further comprise recurring units having the generalformula (U-3) and/or (U-4) as main constituent units.

Herein f is an integer of 0 to 6, R³ is each independently hydrogen, anoptionally halo-substituted C₁-C₆ alkyl or primary or secondary alkoxygroup, or an optionally halo-substituted C₁-C₇ alkylcarbonyloxy group, gis an integer of 0 to 4, and R⁴ is each independently hydrogen, anoptionally halo-substituted C₁-C₆ alkyl or primary or secondary alkoxygroup, or an optionally halo-substituted C₁-C₇ alkylcarbonyloxy group.

Where recurring units of at least one type selected from units offormula (U-3) and units of formula (U-4) are incorporated, the polymermay have another advantage that the binding of cyclic structure to thebackbone enhances resistance to EB (to be irradiated during etching andpattern inspection), in addition to the advantage of etch resistanceinherent to aromatic ring.

In the polymer, recurring units other than the units (U-1) to (U-4) maybe incorporated. For example, (meth)acrylate units protected with anacid labile group as mentioned above and/or (meth)acrylate units havingan adhesive group such as lactone structure may be used.

The polymer used herein may be prepared by any well-known techniques, byselecting suitable monomers and effecting copolymerization whileoptionally combining protection and deprotection reactions. Thecopolymerization reaction is preferably radical or anionicpolymerization though not limited thereto. Reference may be made to WO2006/121096, JP-A 2008-102383, JP-A 2008-304590, and JP-A 2004-115630.

The polymer used herein preferably has a weight average molecular weight(Mw) of 2,000 to 50,000, and more preferably 3,000 to 20,000, asmeasured by GPC using polystyrene standards. As is well known in theart, a polymer with a Mw of at least 2,000 avoids the phenomenon that apattern is rounded at the top, reduced in resolution, and degraded inLER. If Mw is higher than the necessity, there is a tendency ofincreasing LER, depending on a particular pattern to be resolved. Thusthe polymer is preferably controlled to a Mw of up to 50,000, and morepreferably to a Mw of up to 20,000 particularly when a pattern with aline width of up to 100 nm is to be formed. Notably, the GPC measurementmay use tetrahydrofuran (THF) solvent as commonly used.

The polymer used herein should preferably have a narrow dispersity(Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.8. A polymer with narrowdispersity avoids the phenomenon that foreign particles are left on thepattern after development or the pattern is degraded in profile.

In the polymer used herein, recurring units derived from monomers arepreferably incorporated in the following molar fraction (mol %),although the invention is not limited thereto. The polymer preferablycomprises:

-   (I) 35 to 94 mol %, more preferably 40 to 90 mol %, and even more    preferably 55 to 85 mol % of units of at least one type selected    from constituent units having formula (U-1);-   (II) 5 to 45 mol %, more preferably 5 to 40 mol %, and even more    preferably 5 to 30 mol % of units of at least one type selected from    constituent units having formula (U-2); optionally,-   (III) 1 to 20 mol %, more preferably 5 to 20 mol %, and even more    preferably 5 to 15 mol % of units of at least one type selected from    constituent units having formulae (U-3) and (U-4); and optionally,-   (IV) 0 to 30 mol %, more preferably 0 to 20 mol %, and even more    preferably 0 to 15 mol % of units of at least one type selected from    constituent units derived from other monomers.

Preferably, a photoacid generator (PAG) is added to the resistcomposition in order that the composition function as a chemicallyamplified resist composition, especially chemically amplified positiveresist composition. The PAG may be any compound capable of generating anacid upon exposure to high-energy radiation. Preferred is a PAG capableof generating an acid selected from sulfonic acids, imidic acids, andmethide acids, upon exposure to high-energy radiation. Suitable PAGsinclude sulfonium salts, iodonium salts, sulfonyldiazomethane,N-sulfonyloxyimide, and oxime-O-sulfonate compounds, which may be usedalone or in admixture of two or more.

Sulfonium salts are salts of sulfonium cations with anions which aretypically selected from sulfonates, bis(substituted alkylsulfonyl)imidesand tris(substituted alkylsulfonyl)methides. Exemplary sulfonatesinclude trifluoromethanesulfonate, pentafluoroethanesulfonate,heptafluoropropanesulfonate, nonafluorobutanesulfonate,tridecafluorohexanesulfonate, 2,2,2-trifluoroethanesulfonate,pentafluorobenzenesulfonate, 1,1-difluoro-2-naphthylethanesulfonate,1,1,2,2-tetrafluoro-2-(norbornan-2-yl)ethanesulfonate,1,1,2,2-tetrafluoro-2-(tetracyclo[6.2.1.1^(3,6).0^(2,7)]dodec-3-en-8-yl)ethanesulfonate,2-benzoyloxy-1,1,3,3,3-pentafluoropropanesulfonate,1,1-difluoro-2-tosyloxyethanesulfonate,adamantanemethoxycarbonyldifluoromethanesulfonate,1-(3-hydroxymethyladamantane)methoxycarbonyldifluoromethanesulfonate,methoxycarbonyldifluoromethanesulfonate,1-(hexahydro-2-oxo-3,5-methano-2H-cyclopenta[b]furan-6-yloxycarbonyl)difluoromethanesulfonate,and 4-oxo-1-adamantyloxycarbonyldifluoromethanesulfonate. Exemplarybis(substituted alkylsulfonyl)imides includebis(trifluoromethylsulfonyl)imide, bis(pentafluoroethylsulfonyl)imide,bis(heptafluoropropylsulfonyl)imide, andperfluoro(1,3-propylenebissulfonyl)imide. A typical tris(substitutedalkylsulfonyl)methide is tris(trifluoromethylsulfonyl)methide. Sulfoniumsalts based on combination of these anions with the aforementionedcations are included.

Of the above-mentioned acid generators, PAGs of aryl or alkane-sulfonatetype are preferred because they generate acids having an appropriateacid strength to deprotect the acid labile group on the acid labilegroup-bearing units having formula (U-2). The acid generator istypically used in an amount of 0.1 to 40 parts, more preferably 1 to 20parts by weight per 100 parts by weight of the base resin.

A basic compound may be added to the chemically amplified resistcomposition. The addition of a basic compound is effective forcontrolling acid diffusion. The basic compound is typically used in anamount of 0.01 to 5 parts, more preferably 0.05 to 3 parts by weight per100 parts by weight of the base resin. A number of basic compounds areknown. Suitable basic compounds include primary, secondary, and tertiaryaliphatic amines, mixed amines, aromatic amines, heterocyclic amines,nitrogen-containing compounds with carboxyl group, nitrogen-containingcompounds with sulfonyl group, nitrogen-containing compounds withhydroxyl group, nitrogen-containing compounds with hydroxyphenyl group,alcoholic nitrogen-containing compounds, amide derivatives, imidederivatives, carbamate derivatives, and ammonium salts. Numerousexamples of these compounds are described in Patent Document 6. Anybasic compounds may be used alone or in admixture of two or more. Interalia, tris(2-(methoxymethoxy)ethyl)amine,tris(2-(methoxymethoxy)ethyl)amine N-oxide, morpholine derivatives, andimidazole derivatives are preferred.

A surfactant may be added to the chemically amplified resistcomposition. Any suitable one may be selected from those surfactantscommonly used for facilitating coating operation. A number of suitablesurfactants are known, for example, from WO 2006/121096, JP-A2008-102383, JP-A 2008-304590, JP-A 2004-115630, and JP-A 2005-008766.The surfactant is typically used in an amount of up to 2 parts, morepreferably 0.01 to 1 part by weight per 100 parts by weight of the baseresin.

Process

Another embodiment of the invention is a pattern forming process usingthe resist composition defined above. A pattern may be formed from theresist composition using any well-known lithography process. Thepreferred process includes at least the steps of forming a resist filmon a substrate, exposing it patternwise to high-energy radiation, anddeveloping it in an alkaline developer. The resist composition isapplied onto a substrate for integrated circuit fabrication (e.g., Si,SiO₂, SiN, SiON, TiN, WSi, BPSG, SOG or organic antireflective coating)or a substrate for mask circuit fabrication (e.g., Cr, CrO, CrON orMoSi) by a suitable coating technique such as spin coating. The coatingis prebaked on a hot plate at a temperature of 60 to 150° C. for 1 to 20minutes, preferably 80 to 140° C. for 1 to 10 minutes, to form a resistfilm of 0.05 to 2.0 μm thick.

The resist film is then exposed by the lithography. Specifically, theresist film is subjected to direct beam writing or exposed tohigh-energy radiation such as DUV, excimer laser, X-ray or EB through amask having the desired pattern in a dose of 1 to 200 mJ/cm², andpreferably 10 to 100 mJ/cm². The chemically amplified resist compositionof the invention is best suited for pattern imaging with EUV or EB. Theexposure step may be performed by standard lithography. If desired, theimmersion lithography using a liquid, typically water between the maskand the resist film is applicable. In this case, a protective film whichis insoluble in water may be formed on the resist film.

After exposure, the resist film is baked (PEB) on a hot plate at 60 to150° C. for 1 to 20 minutes, and preferably at 80 to 140° C. for 1 to 10minutes. This is followed by development in a developer which is analkaline aqueous solution, typically an aqueous solution of 0.1 to 5 wt%, more typically 2 to 3 wt % of tetramethylammonium hydroxide (TMAH).Development may be carried out by a conventional method such as dip,puddle, or spray development for 0.1 to 3 minutes, and preferably 0.5 to2 minutes. These steps result in the formation of the desired positivepattern on the substrate.

One advantage of the resist composition is high etch resistance. Alsothe resist composition is effective when it is required that the patternexperience a minimal change of line width with a post-exposure delay(PED), i.e., when the duration between exposure and PEB is prolonged.The resist composition is effectively applicable to a processablesubstrate, specifically a substrate having a surface layer of materialto which a resist film is less adherent and which is likely to invitepattern stripping or pattern collapse, and particularly a substratehaving sputter deposited thereon metallic chromium or a chromiumcompound containing at least one light element selected from oxygen,nitrogen and carbon. The invention is effective for pattern formation onphotomask blanks.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation. Mw and Mn are weight and number average molecularweights, respectively, as measured by GPC versus polystyrene standards,and Mw/Mn is a polydispersity index. Me stands for methyl.

Synthesis Example 1 Synthesis of Carboxylic Acid Sulfonium Salt

Carboxylic acid sulfonium salts were synthesized by the followingprocedure.

Synthesis Example 1-1 Synthesis of 2,4,6-triisopropylbenzoic acid(Intermediate #1)

A Grignard reagent was prepared from 56.7 g of1-bromo-2,4,6-triisopropylbenzene and poured to 300 g of dry ice,whereupon dry ice sublimated. Hydrochloric acid was added to thereaction mixture for quenching, followed by extraction with 200 g oftoluene. The organic layer was taken out, washed with water, andcombined with 10 wt % sodium hydroxide aqueous solution. The aqueouslayer was taken out, combined with 50 g of 35 wt % hydrochloric acid,and extracted with 200 g of toluene. The organic layer was washed withwater again, and concentrated under reduced pressure. The concentratewas crystallized from 150 g of hexane. The crystal precipitate wasfiltered and dried, obtaining the target compound,2,4,6-triisopropylbenzoic acid. Amount 39.7 g, yield 80%.

Synthesis Example 1-2 Synthesis of sodium 2,4,6-triisopropylbenzoate(Intermediate #2)

A 1-L eggplant shaped flask was charged with 39.7 g of2,4,6-triisopropylbenzoic acid (prepared in Synthesis Example 1-1), 13.4g of sodium hydrogencarbonate, 160 g of methyl isobutyl ketone, and 40 gof water, which were stirred at room temperature for 24 hours. At theend of stirring, the reaction solution was concentrated, followed bytwice of azeotropic dehydration with 100 g of methyl isobutyl ketone. Tothe solid precipitate, 300 g of diisopropyl ether was added, whereuponsolid-liquid washing was conducted for 30 minutes. The solid wasfiltered and dried, obtaining the target compound, sodium2,4,6-triisopropylbenzoate as white crystal. Amount 30.7 g, yield 80%.

Synthesis Example 1-3 Synthesis of triphenylsulfonium2,4,6-triisopropylbenzoate (Salt-1)

A 1-L eggplant shaped flask was charged with 22.4 g of sodium2,4,6-triisopropylbenzoate (prepared in Synthesis Example 1-2), 31.1 gof triphenylsulfonium methylsulfate, 90 g of methyl isopropyl ketone, 90g of 1-pentanol, and 40 g of water, which were stirred at roomtemperature for 1 hour. The organic layer was separated from thereaction mixture and washed with water. After washing, the organic layerwas concentrated by distilling off the solvent. The liquid concentratewas crystallized from 300 g of diisopropyl ether. The crystalprecipitate was filtered and dried, obtaining the target compound,triphenylsulfonium 2,4,6-triisopropylbenzoate as white crystal. Amount40.4 g, yield 80%.

Synthesis Example 1-4 Synthesis of 2-bromo-1,3,5-tricyclohexylbenzene(Intermediate #3)

A 500-mL three-neck flask was charged with 15.7 g of N-bromosuccinimideand 100 g of acetonitrile. Under ice cooling, a dilute solution of 28.4g of 1,3,5-tricyclohexylbenzene (prepared by known reaction) in 60 g ofmethylene chloride was added dropwise over 30 minutes. The solution waswarmed to room temperature and aged for 12 hours. The resulting crystalprecipitate was filtered, washed with acetonitrile and water, and dried,obtaining the target compound, 2-bromo-1,3,5-tricyclohexylbenzene aswhite crystal. Amount 28.4 g, yield 81%.

Synthesis Example 1-5 Synthesis of 2,4,6-tricyclohexylbenzoic acid(Intermediate #4)

A Grignard reagent was prepared from 28.4 g of2-bromo-1,3,5-tricyclohexylbenzene (prepared in Synthesis Example 1-4)and poured to 300 g of dry ice, whereupon dry ice sublimated.Hydrochloric acid was added to the reaction mixture for quenching,followed by extraction with 200 g of isopropyl ether. The organic layerwas taken out, washed with water, and combined with 150 g of 10 wt %sodium hydroxide aqueous solution. The aqueous layer was taken out,combined with 100 g of 35 wt % hydrochloric acid, and extracted with 200g of diisopropyl ether. The organic layer was washed with water again,and concentrated under reduced pressure. The concentrate wascrystallized from 150 g of hexane. The crystal precipitate was filteredand dried, obtaining the target compound, 2,4,6-tricyclohexylbenzoicacid. Amount 39.7 g, yield 80%.

Synthesis Example 1-6 Synthesis of sodium 2,4,6-tricyclohexylbenzoate(Intermediate #5)

A 300-mL eggplant shaped flask was charged with 2.0 g of2,4,6-tricyclohexylbenzoic acid (prepared in Synthesis Example 1-5), 0.5g of sodium hydrogencarbonate, 10 g of methyl isobutyl ketone, and 10 gof water, which were stirred at room temperature. At the end ofstirring, the reaction solution was concentrated, followed by twice ofazeotropic dehydration with 30 g of methyl isobutyl ketone. To the solidprecipitate, 30 g of acetonitrile was added, whereupon solid-liquidwashing was conducted for 30 minutes. The solid was filtered and dried,obtaining the target compound, sodium 2,4,6-tricyclohexylbenzoate aswhite crystal. Amount 2.0 g, yield 90%.

Synthesis Example 1-7 Synthesis of triphenylsulfonium2,4,6-tricyclohexylbenzoate (Salt-2)

A 100-mL eggplant shaped flask was charged with 1.0 g of sodium2,4,6-tricyclohexylbenzoate (prepared in Synthesis Example 1-6), 0.9 gof triphenylsulfonium methylsulfate, 10 g of methyl isopropyl ketone, 10g of 1-pentanol, and 5 g of water, which were stirred at roomtemperature for 1 hour. The organic layer was separated from thereaction mixture and washed with water. After washing, the organic layerwas concentrated by distilling off the solvent. The liquid concentratewas crystallized from 30 g of diisopropyl ether. The crystal precipitatewas filtered and dried, obtaining the target compound,triphenylsulfonium 2,4,6-tricyclohexylbenzoate as white crystal. Amount1.2 g, yield 76%.

Synthesis Example 1-8 Synthesis of 10-phenyl-10-phenoxathiiniummethylsulfate (Intermediate #6)

A mixture of 20 g of phenoxathiin, 43.1 g of diphenyliodoniummethylsulfate, 0.9 g of copper(II) benzoate, and 210 g of chlorobenzenewas heated and stirred at 120° C. for 3 hours. The reaction solution wascooled to room temperature and combined with 20 g of diisopropyl etherfor crystallization. The solid was dried under reduced pressure,obtaining the target compound, 10-phenyl-10-phenoxathiiniummethylsulfate. Amount 24.7 g, yield 63%.

Synthesis Example 1-9 Synthesis of 10-phenyl-10-phenoxathiinium2,4,6-tricyclohexylbenzoate (Salt-3)

A 100-mL eggplant shaped flask was charged with 1.0 g of sodium2,4,6-tricyclohexylbenzoate (prepared in Synthesis Example 1-6), 0.9 gof 10-phenyl-10-phenoxathiinium methylsulfate (prepared in SynthesisExample 1-8), 10 g of methyl isopropyl ketone, 10 g of 1-pentanol, and 5g of water, which were stirred at room temperature for 1 hour. Theorganic layer was separated from the reaction mixture and washed withwater. After washing, the organic layer was concentrated by distillingoff the solvent. The liquid concentrate was crystallized from 30 g ofdiisopropyl ether. The crystal precipitate was filtered and dried,obtaining the target compound, 10-phenyl-10-phenoxathiinium2,4,6-tricyclohexylbenzoate as white crystal. Amount 1.4 g, yield 88%.

Synthesis Example 2 Synthesis of Polymers

Polymers for use in resist compositions were synthesized according tothe following formulation. The compositional proportion (in molar ratio)of polymers is shown in Table 1. The structure of recurring units isshown in Tables 2 to 4.

Synthesis Example 2-1 Synthesis of Polymer 1

A 3-L flask was charged with 407.5 g of acetoxystyrene, 42.5 g ofacenaphthylene, and 1,275 g of toluene as solvent. The reactor wascooled at −70° C. in a nitrogen atmosphere, after which vacuum pumpingand nitrogen flow were repeated three times. The reactor was warmed upto room temperature, whereupon 34.7 g of2,2′-azobis(2,4-dimethylvaleronitrile) (V-65 by Wako Pure ChemicalIndustries, Ltd.) was added as polymerization initiator. The reactor washeated at 55° C., whereupon reaction ran for 40 hours. With stirring, amixture of 970 g of methanol and 180 g of water was added dropwise tothe reaction solution. After 30 minutes of standing, the lower layer(polymer layer) was concentrated under reduced pressure. The polymerlayer concentrate was dissolved again in 0.45 L of methanol and 0.54 Lof tetrahydrofuran (THF), to which 160 g of triethylamine and 30 g ofwater were added. The reaction mixture was heated at 60° C. for 40 hoursfor deprotection reaction. The reaction solution was concentrated underreduced pressure. To the concentrate, 548 g of methanol and 112 g ofacetone were added for dissolution. With stirring, 990 g of hexane wasadded dropwise to the solution. After 30 minutes of standing, 300 g ofTHF was added to the lower layer (polymer layer). With stirring, 1,030 gof hexane was added dropwise thereto. After 30 minutes of standing, thelower layer (polymer layer) was concentrated under reduced pressure. Thepolymer solution was neutralized with 82 g of acetic acid. The reactionsolution was concentrated, dissolved in 0.3 L of acetone, and pouredinto 10 L of water for precipitation. The precipitate was filtered anddried, yielding 280 g of a white polymer. The polymer was analyzed by¹H-NMR and GPC, with the results shown below.

Copolymer Compositional Ratio

hydroxystyrene:acenaphthylene=89.3:10.7

Mw=5,000

Mw/Mn=1.63

Under acidic conditions, 100 g of the polymer was reacted with 50 g of2-methyl-1-propenyl methyl ether. This was followed by neutralization,phase separation, and crystallization, obtaining 125 g of a polymer,designated Polymer 1.

Synthesis Example 2-2 Synthesis of Polymer 2

Polymer 2 was synthesized by the same procedure as in Synthesis Example2-1 aside from using 2-methyl-1-propenyl8-tricyclo[5.2.1.0^(2,6)]decanyl ether instead of 2-methyl-1-propenylmethyl ether.

Synthesis Example 2-3 Synthesis of Polymer 3

Polymer 3 was synthesized by the same procedure as in Synthesis Example2-1 aside from using 2-methyl-1-propenyl 2-adamantyl ether instead of2-methyl-1-propenyl methyl ether.

Synthesis Example 2-4 Synthesis of Polymer 4

In nitrogen atmosphere, 362 g of 4-hydroxyphenyl methacrylate, 38.2 g ofacenaphthylene, 40.9 g of dimethyl 2,2′-azobis(2-methylpropionate)(V-601 by Wako Pure Chemical Industries, Ltd.), and 500 g of methylethyl ketone were fed into a dropping cylinder to form a monomersolution. A flask in nitrogen atmosphere was charged with 250 g ofmethyl ethyl ketone, which was heated at 80° C. with stirring. Withstirring, the monomer solution was added dropwise to the flask over 4hours. After the completion of dropwise addition, the polymerizationsolution was continuously stirred for 4 hours while maintaining itstemperature at 80° C. The polymerization solution was cooled to roomtemperature, whereupon it was added dropwise to 10 kg ofhexane/diisopropyl ether solution. The precipitate was collected byfiltration, washed twice with 5 kg of hexane, and vacuum dried at 50° C.for 20 hours, obtaining a copolymer in white powder solid form. Underacidic conditions, 100 g of the polymer was reacted with 40.5 g of2-methyl-1-propenyl methyl ether. This was followed by neutralization,phase separation, and crystallization, obtaining 128 g of a polymer,designated Polymer 4.

Synthesis Example 2-5 Synthesis of Polymer 5

Polymer 5 was synthesized by the same procedure as in Synthesis Example2-4 aside from using 2-methyl-1-propenyl8-tricyclo[5.2.1.0^(2,6)]decanyl ether instead of 2-methyl-1-propenylmethyl ether.

Synthesis Example 2-6 Synthesis of Polymer 6

Polymer 6 was synthesized by the same procedure as in Synthesis Example2-4 aside from using 2-methyl-1-propenyl 2-adamantyl ether instead of2-methyl-1-propenyl methyl ether.

Synthesis Examples 2-7 to 2-12 Synthesis of Polymers 7 to 12

Polymers containing hydroxystyrene units in Table 1 were synthesized bythe same procedure as in Synthesis Example 2-1, 2-2 or 2-3 aside fromchanging the type and amount of monomers. Polymers containing4-hydroxyphenyl methacrylate units in Table 1 were synthesized by thesame procedure as in Synthesis Example 2-4, 2-5 or 2-6 aside fromchanging the type and amount of monomers.

Synthesis Example 2-13 Synthesis of Polymer 13

In nitrogen atmosphere, 42.4 g of 4-hydroxyphenyl methacrylate, 40.6 gof 5-oxo-4-oxatricyclo[4.2.1.0^(3,7)]nonan-2-yl methacrylate, 16.9 g of1-methoxy-2-methyl-1-propyl methacrylate, 9.3 g of dimethyl2,2′-azobis(2-methylpropionate) (V-601 by Wako Pure Chemical Industries,Ltd.), and 124 g of methyl ethyl ketone were fed into a droppingcylinder to form a monomer solution. A flask in nitrogen atmosphere wascharged with 62 g of methyl ethyl ketone, which was heated at 80° C.with stirring. With stirring, the monomer solution was added dropwise tothe flask over 4 hours. After the completion of dropwise addition, thepolymerization solution was continuously stirred for 4 hours whilemaintaining its temperature at 80° C. The polymerization solution wascooled to room temperature, whereupon it was added dropwise to 1.5 kg ofhexane/diisopropyl ether solution. The precipitate was collected byfiltration, washed twice with 300 g of hexane, and vacuum dried at 50°C. for 20 hours, obtaining a copolymer in white powder solid form. It isdesignated Polymer 13.

Synthesis Examples 2-14 and 2-15 Synthesis of Polymers 14 and 15

Polymers in Table 1 were synthesized by the same procedure as inSynthesis Example 2-13 aside from changing the type and amount ofmonomers.

Synthesis Example 2-16 Synthesis of Polymer 16

In nitrogen atmosphere, 64.8 g of 4-acetoxystyrene, 9.1 g ofacenaphthylene, 26.1 g of amyloxystyrene, 11.0 g of dimethyl2,2′-azobis(2-methylpropionate) (V-601 by Wako Pure Chemical Industries,Ltd.), and 150 g of methyl ethyl ketone were fed into a droppingcylinder to form a monomer solution. A flask in nitrogen atmosphere wascharged with 75 g of methyl ethyl ketone, which was heated at 80° C.with stirring. With stirring, the monomer solution was added dropwise tothe flask over 4 hours. After the completion of dropwise addition, thepolymerization solution was continuously stirred for 18 hours whilemaintaining its temperature at 80° C. The polymerization solution wascooled to room temperature, whereupon it was added dropwise to 1.5 kg ofhexane/diisopropyl ether solution. The precipitate was collected byfiltration and washed twice with 300 g of hexane. The copolymer wasdissolved in 180 g of THF and 60 g of methanol, to which 24.4 g ofethanol amine was added. Under reflux, the solution was stirred for 3hours. The reaction solution was concentrated under reduced pressure anddissolved in ethyl acetate. This was followed by neutralization, phaseseparation, and crystallization, obtaining Polymer 16. Amount 71 g.

Table 1 shows the proportion (in molar ratio) of units incorporated inthese polymers, and Tables 2 to 4 show the structure of recurring units.

TABLE 1 Unit Proportion Unit Proportion Proportion 1 (mol %) 2 (mol %)Unit 3 (mol %) Polymer 1 A-1 70.0 B-1 20.0 C-1 10.0 Polymer 2 A-1 78.0B-3 12.0 C-1 10.0 Polymer 3 A-1 79.0 B-5 11.0 C-1 10.0 Polymer 4 A-267.0 B-2 23.0 C-1 10.0 Polymer 5 A-2 76.0 B-4 14.0 C-1 10.0 Polymer 6A-2 77.0 B-6 13.0 C-1 10.0 Polymer 7 A-1 68.0 B-1 22.0 C-2 10.0 Polymer8 A-1 76.0 B-3 14.0 C-2 10.0 Polymer 9 A-1 77.0 B-5 13.0 C-2 10.0Polymer 10 A-2 64.0 B-2 26.0 C-2 10.0 Polymer 11 A-2 73.0 B-4 17.0 C-210.0 Polymer 12 A-2 74.0 B-6 16.0 C-2 10.0 Polymer 13 A-2 46.0 B-7 19.0C-3 35.0 Polymer 14 A-2 50.0 B-8 15.0 C-3 35.0 Polymer 15 A-2 50.0 B-915.0 C-3 35.0 Polymer 16 A-1 67.0 B-10 23.0 C-1 10.0

TABLE 2

A-1

A-2

TABLE 3

B-1

B-2

B-3

B-4

B-5

B-6

B-7

B-8

B-9

B-10

TABLE 4

C-1

C-2

C-3Preparation of Positive Resist Composition(A) Acid Diffusion Regulator:

-   -   inventive salts (Salt-1 to Salt-3) or comparative salts        (Comparative Salt-1 to Comparative Salt-6) of the structure        shown in Table 6        (B) Polymer:    -   polymers synthesized above (Polymer 1 to Polymer 16)        (C) Photoacid Generator:    -   salts (PAG-1 to PAG-4) of the structure shown in Table 5

A positive resist composition in solution form was prepared bydissolving the components in an organic solvent in accordance with theformulation shown in Tables 7 and 8, and filtering through a filter witha pore size of 0.2 μm or a nylon or UPE filter with a pore size of 0.02μm. The organic solvents in Tables 7 and 8 are PGMEA (propylene glycolmonomethyl ether acetate), EL (ethyl lactate), PGME (propylene glycolmonomethyl ether), and CyH (cyclohexanone). The composition contained0.075 part of surfactant PF-636 (Omnova Solutions Inc.).

TABLE 5

PAG-1

PAG-2

PAG-3

PAG-4

TABLE 6

Salt-1

Salt-2

Salt-3

Comparative Salt-1

Comparative Salt-2

Comparative Salt-3

Comparative Salt-4

Comparative Salt-5

Comparative Salt-6

TABLE 7 Acid diffusion regulator Polymer PAG Solvent 1 Solvent 2 Solvent3 (pbw) (pbw) (pbw) (pbw) (pbw) (pbw) Example 1 Salt-1(2.0) Polymer1(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 2 Salt-1(2.0) Polymer2(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 3 Salt-1(2.1) Polymer3(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 4 Salt-2(2.2) Polymer1(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 5 Salt-2(2.2) Polymer2(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 6 Salt-2(2.1) Polymer3(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 7 Salt-3(2.1) Polymer1(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 8 Salt-3(2.2) Polymer2(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 9 Salt-3(2.2) Polymer3(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 10 Salt-1(2.3) Polymer2(80) PAG-4(10) PGMEA(1,000) EL(1,000) PGME(1,300) 11 Salt-1(2.4)Polymer 3(80) PAG-4(10) PGMEA(1,000) EL(1,000) PGME(1,300) 12Salt-2(2.5) Polymer 2(80) PAG-4(10) PGMEA(1,000) EL(1,000) PGME(1,300)13 Salt-2(2.6) Polymer 3(80) PAG-4(10) PGMEA(1,000) EL(1,000)PGME(1,300) 14 Salt-3(2.6) Polymer 2(80) PAG-4(10) PGMEA(1,000)EL(1,000) PGME(1,300) 15 Salt-3(2.6) Polymer 3(80) PAG-4(10)PGMEA(1,000) EL(1,000) PGME(1,300) 16 Salt-1(2.5) Polymer 2(80) PAG-2(5)PGMEA(1,000) EL(1,000) PGME(1,300) PAG-3(5) 17 Salt-1(2.7) Polymer 2(80)PAG-1(10) PGMEA(1,000) EL(1,000) PGME(1,300) 18 Salt-1(2.9) Polymer2(80) PAG-3(12) PGMEA(1,000) EL(1,000) PGME(1,300) 19 Salt-1(3.0)Polymer 2(80) PAG-1(12) PGMEA(1,000) EL(1,000) PGME(1,300) 20Salt-1(2.6) Polymer 3(80) PAG-3(10) PGMEA(1,000) EL(1,000) PGME(1,300)21 Salt-1(2.7) Polymer 3(80) PAG-2(10) PGMEA(1,000) EL(1,000)PGME(1,300) 22 Salt-1(2.6) Polymer 3(80) PAG-2(5) PGMEA(1,000) EL(1,000)PGME(1,300) PAG-3(5) 23 Salt-1(3.0) Polymer 3(80) PAG-3(12) PGMEA(1,000)EL(1,000) PGME(1,300) 24 Salt-1(3.2) Polymer 3(80) PAG-2(12)PGMEA(1,000) EL(1,000) PGME(1,300) 25 Salt-1(2.1) Polymer 4(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) 26 Salt-1(2.1) Polymer 5(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) 27 Salt-1(2.1) Polymer 6(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) 28 Salt-1(2.5) Polymer 7(80)PAG-2(10) PGMEA(1,000) EL(1,000) PGME(1,300) 29 Salt-1(2.2) Polymer8(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 30 Salt-1(2.3) Polymer9(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300)

TABLE 8 Acid diffusion regulator Polymer PAG Solvent 1 Solvent 2 Solvent3 (pbw) (pbw) (pbw) (pbw) (pbw) (pbw) Example 31 Salt-1(2.6) Polymer10(80) PAG-2(10) PGMEA(1,000) EL(1,000) PGME(1,300) 32 Salt-1(2.3)Polymer 11(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) 33Salt-1(2.4) Polymer 12(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300)34 Salt-1(2.5) Polymer 2(80) PAG-3(8) PGMEA(1,000) EL(1,000) PGME(1,300)35 Salt-1(2.4) Polymer 3(80) PAG-3(8) PGMEA(1,000) EL(1,000) PGME(1,300)36 Salt-2(2.6) Polymer 2(80) PAG-2(5) PGMEA(1,000) EL(1,000) PGME(1,300)PAG-3(5) 37 Salt-3(2.7) Polymer 3(80) PAG-2(5) PGMEA(1,000) EL(1,000)PGME(1,300) PAG-3(5) 38 Salt-1(2.4) Polymer 8(80) PAG-3(8) PGMEA(1,000)EL(1,000) PGME(1,300) 39 Salt-1(2.2) Polymer 9(80) PAG-3(8) PGMEA(1,000)EL(1,000) PGME(1,300) 40 Salt-1(2.3) Polymer 8(80) PAG-1(8) PGMEA(1,000)EL(1,000) PGME(1,300) 41 Salt-1(2.3) Polymer 9(80) PAG-1(8) PGMEA(1,000)EL(1,000) PGME(1,300) 42 Salt-1(1.8) Polymer 16(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) 43 Salt-1(1.8) Polymer 13(80)PAG-4(8) PGMEA(800) CyH(1,600) PGME(400) 44 Salt-1(1.8) Polymer 14(80)PAG-4(8) PGMEA(800) CyH(1,600) PGME(400) 45 Salt-1(1.8) Polymer 15(80)PAG-4(8) PGMEA(800) CyH(1,600) PGME(400) Comparative 1 ComparativePolymer 2(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) ExampleSalt-1(1.8) 2 Comparative Polymer 3(80) PAG-4(8) PGMEA(1,000) EL(1,000)PGME(1,300) Salt-1(1.9) 3 Comparative Polymer 2(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) Salt-2(1.6) 4 Comparative Polymer3(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) Salt-2(1.6) 5Comparative Polymer 2(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300)Salt-3(1.8) 6 Comparative Polymer 3(80) PAG-4(8) PGMEA(1,000) EL(1,000)PGME(1,300) Salt-3(1.8) 7 Comparative Polymer 2(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) Salt-4(1.2) 8 Comparative Polymer3(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) Salt-4(1.2) 9Comparative Polymer 2(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300)Salt-5(1.8) 10 Comparative Polymer 3(80) PAG-4(8) PGMEA(1,000) EL(1,000)PGME(1,300) Salt-5(1.8) 11 Comparative Polymer 2(80) PAG-4(8)PGMEA(1,000) EL(1,000) PGME(1,300) Salt-6(1.8) 12 Comparative Polymer3(80) PAG-4(8) PGMEA(1,000) EL(1,000) PGME(1,300) Salt-6(1.8) 13Comparative Polymer 13(80) PAG-4(8) PGMEA(800) CyH(1,600) PGME(400)Salt-1(1.9) 14 Comparative Polymer 13(80) PAG-4(8) PGMEA(800) CyH(1,600)PGME(400) Salt-2(1.6) 15 Comparative Polymer 13(80) PAG-4(8) PGMEA(800)CyH(1,600) PGME(400) Salt-3(1.8) 16 Comparative Polymer 13(80) PAG-4(8)PGMEA(800) CyH(1,600) PGME(400) Salt-4(1.2) 17 Comparative Polymer13(80) PAG-4(8) PGMEA(800) CyH(1,600) PGME(400) Salt-5(1.8) 18Comparative Polymer 13(80) PAG-4(8) PGMEA(800) CyH(1,600) PGME(400)Salt-6(1.8)

Examples 1 to 42 and Comparative Examples 1 to 12 EB Writing Test

Using a coater/developer system ACT-M (Tokyo Electron Ltd.), each of thepositive resist compositions (prepared above as Examples 1 to 42 andComparative Examples 1 to 12) was spin coated onto a mask blank of 152mm squares having a chromium oxynitride film at the outermost surfaceand prebaked on a hot plate at 120° C. for 600 seconds to form a resistfilm of 90 nm thick. The thickness of the resist film was measured by anoptical film thickness measurement system Nanospec (Nanometrics Inc.).Measurement was made at 81 points in the plane of the blank substrateexcluding a peripheral band extending 10 mm inward from the blankperiphery, and an average film thickness and a film thickness range werecomputed therefrom.

The coated mask blanks were exposed to electron beam using an EB writersystem EBM-5000Plus (NuFlare Technology Inc., accelerating voltage 50keV), then baked (PEB) at 120° C. for 600 seconds, and developed in a2.38 wt % tetramethylammonium hydroxide (TMAH) aqueous solution, therebyyielding positive patterns.

The patterned mask blank was observed under a top-down scanning electronmicroscope (TDSEM). The optimum exposure (Eop) was defined as theexposure dose (μC/cm²) which provided a 1:1 resolution at the top andbottom of a 400-nm 1:1 line-and-space pattern. The maximum resolution ofthe resist was defined as the minimum line width of a line-and-spacepattern that could be resolved at the optimum exposure. The LER of a200-nm line-and-space pattern was measured under SEM. On observation incross section of the resist pattern under SEM, it was visually judgedwhether or not the pattern profile was rectangular.

For evaluation of CDU, the line width of the pattern at the optimumexposure Eop (μC/cm²) (which provided a 1:1 resolution of a 400-nm 1:1line-and-space pattern) was measured at 49 points in the plane of theblank substrate excluding a peripheral band extending 20 mm inward fromthe blank periphery. A 3σ value was computed by subtracting the width ateach measurement point from the average line width, and reported as CDU.Tables 9 and 10 show the results of evaluation of inventive andcomparative resist compositions as patterned by EB writing.

TABLE 9 Maximum Eop, resolution, LER, CDU (3σ), Pattern μC/cm² nm nm nmprofile Example 1 21 40 4.9 2.5 rectangular 2 24 40 4.8 2.4 rectangular3 25 40 4.9 2.4 rectangular 4 21 40 4.8 2.5 rectangular 5 25 40 4.6 2.4rectangular 6 20 40 4.7 2.5 rectangular 7 24 40 4.7 2.5 rectangular 8 2040 4.8 2.4 rectangular 9 24 40 4.6 2.1 rectangular 10 22 40 4.9 2.2rectangular 11 21 40 4.8 2.2 rectangular 12 21 45 4.6 2.3 rectangular 1325 40 4.7 2.3 rectangular 14 23 45 4.8 2.1 rectangular 15 20 40 4.6 2.5rectangular 16 22 45 4.6 2.3 rectangular 17 21 45 4.6 2.3 rectangular 1824 40 4.6 2.4 rectangular 19 23 45 4.8 2.4 rectangular 20 25 45 4.6 2.5rectangular 21 25 40 4.8 2.1 rectangular 22 20 40 5.0 2.4 rectangular 2320 45 4.8 2.1 rectangular 24 23 40 5.0 2.4 rectangular 25 22 45 4.6 2.5rectangular 26 22 40 4.8 2.2 rectangular 27 24 45 5.0 2.1 rectangular 2823 45 4.8 2.5 rectangular 29 23 45 4.8 2.3 rectangular 30 23 40 4.5 2.2rectangular

TABLE 10 Eop, Maximum μC/ resolution, LER, CDU (3σ), Pattern cm² nm nmnm profile Example 31 22 40 4.7 2.4 rectangular 32 25 40 5.0 2.1rectangular 33 24 40 4.6 2.2 rectangular 34 24 45 4.8 2.3 rectangular 3524 45 4.7 2.2 rectangular 36 25 45 4.8 2.5 rectangular 37 24 45 4.7 2.3rectangular 38 25 45 4.9 2.5 rectangular 39 24 40 4.8 2.4 rectangular 4023 40 4.7 2.3 rectangular 41 25 45 4.8 2.4 rectangular 42 25 45 4.6 2.5rectangular Comparative 1 24 55 5.6 3.5 rectangular Example 2 23 55 6.73.4 rectangular 3 25 55 7.2 3.4 rectangular 4 26 55 7.1 3.6 rectangular5 24 55 6.7 3.7 rectangular 6 25 55 7.1 3.6 rectangular 7 27 50 6.6 3.7rectangular 8 26 50 6.4 3.9 rectangular 9 7 65 8.8 4.5 tapered 10 8 658.9 4.8 tapered 11 6 65 8.7 2.1 tapered 12 7 65 8.5 5.0 tapered

Examples 43 to 45 and Comparative Examples 13 to 18 EUV Exposure Test

Each of the positive resist compositions (prepared above as Examples 43to 45 and Comparative Examples 13 to 18) was spin coated on a siliconsubstrate (diameter 4 inches, vapor primed with hexamethyldisilazane(HMDS)) and prebaked on a hot plate at 105° C. for 60 seconds to form aresist film of 50 nm thick. EUV exposure was performed by dipoleillumination at NA 0.3. Immediately after the exposure, the resist filmwas baked (PEB) on a hot plate for 60 seconds and puddle developed in a2.38 wt % TMAH aqueous solution for 30 seconds to form a positivepattern.

The optimum exposure (Eop) is defined as the exposure dose that providesa 1:1 resolution of a 35-nm line-and-space pattern. Maximum resolutionis a minimum size that can be resolved at Eop. The 35-nm line-and-spacepattern was measured for LER under SEM. On observation in cross sectionof the resist pattern under SEM, it was visually judged whether or notthe pattern profile was rectangular. The results of the resistcompositions by EUV lithography test is shown in Table 11.

TABLE 11 Maximum Eop, resolution, LER, mJ/cm² nm nm Pattern profileExample 43 15 28 4.0 rectangular 44 16 26 4.1 rectangular 45 17 26 4.3rectangular Comparative 13 12 50 7.5 tapered Example 14 12 55 7.3tapered 15 14 50 7.2 tapered 16 13 50 6.9 tapered 17 6 65 8.5 tapered 186 65 8.9 tapered

As seen from the results in Tables 9, 10 and 11, the resist compositionscontaining the sulfonium salt of formula (1) within the scope of theinvention (Examples 1 to 42 and Examples 43 to 45) as acid diffusionregulator exhibit a high resolution, satisfactory patternrectangularity, and acceptable values of LER. In contrast, the resistcompositions containing acid diffusion regulators of benzoic acid typehaving only one bulky substituent group or relatively low methyl assubstituent group or of salicylic acid type (Comparative Examples 1 to18) are inferior in resolution, PED, CDU, and LER. The resistcompositions containing acid diffusion regulators of fluorocarboxylicacid type and pentafluorobenzoic acid type form patterns of taperedprofile.

Although the reason is not well understood, the carboxylic acidsulfonium salt of the invention is characterized in that the anionmoiety has a bulky structure of arenecarboxylate in which secondary ortertiary carbon atoms bond at both ortho-positions relative to thecarbon atom in bond with carboxylate. Thus the inventive onium salt ismore lipophilic than the comparative salts and fully compatible withresist components. Although the weak acid onium salt is generallyinsufficient to cleave the acid labile group on the base resin, there isstill a possibility that it functions as a photoacid generator in afully exposed region. Since the inventive onium salt has sterichindrance around carboxylate as pointed out above, the carboxylic acidgenerated by the onium salt does not react with the acid labile group onthe base resin and purely functions only as the acid diffusionregulator. As a result, it improves the lithography performance,especially LER, PED and CDU over the comparative onium salts. The aciddiffusion regulators of benzoic acid type (Comparative Examples 1 to 8and 13 to 16) are so water soluble that they may penetrate into theunexposed region during alkaline development, whereby performancefactors typically LER are degraded. The acid diffusion regulators ofComparative Examples 9 to 12, 17 and 18 generate carboxylic acids havinga sufficiently high acidity to react with the acid labile group on thebase resin, whereby the performance factors and pattern profile aredegraded.

It has been demonstrated that using the resist composition within thescope of the invention, a pattern having minimal LER can be formed viaexposure. The pattern forming process using the resist compositionwithin the scope of the invention is advantageous in thephotolithography for semiconductor device fabrication and photomaskblank processing.

While the invention has been illustrated and described in typicalembodiments, it is not intended to be limited to the details shown. Anymodified embodiments having substantially the same features andachieving substantially the same results as the technical idea disclosedherein are within the spirit and scope of the invention.

Japanese Patent Application No. 2014-118577 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

The invention claimed is:
 1. A chemically amplified resist compositioncomprising (A) a sulfonium salt having the general formula (1) and (B) apolymer comprising recurring units having the general formula (U-1),which is decomposed under the action of acid to increase its solubilityin alkaline developer,

wherein R¹¹ and R²² are each independently a branched or cyclic C₃-C₂₀monovalent hydrocarbon group which may be substituted with or separatedby a heteroatom, R³³ and R⁰¹ are each independently hydrogen, or astraight C₁-C₂₀ or branched or cyclic C₃-C₂₀ monovalent hydrocarbongroup which may be substituted with or separated by a heteroatom, k isan integer of 0 to 4, or R¹¹, R²², R³³ and R⁰¹ may bond together to forma ring with the carbon atoms to which they are attached and the carbonatom or atoms therebetween, m is 0 or 1, R¹⁰¹, R¹⁰² and R¹⁰³ are eachindependently a straight C₁-C₂₀ or branched or cyclic C₃-C₂₀ monovalenthydrocarbon group which may be substituted with or separated by aheteroatom, or any two or more of R¹⁰¹, R¹⁰² and R¹⁰³ may bond togetherto form a ring with the sulfur atom,

wherein q is 0 or 1, r is an integer of 0 to 2, R¹ is hydrogen,fluorine, methyl or trifluoromethyl, R² is each independently hydrogenor C₁-C₆ alkyl, B¹ is a single bond or C₁-C₁₀ alkylene which may containan ether bond, a is an integer satisfying a≦5+2r−b, and b is an integerof 1 to
 3. 2. The resist composition of claim 1 wherein component (A) isselected from the following sulfonium salts (A-30) to (A-44).


3. The resist composition of claim 1, further comprising an acidgenerator capable of generating at least one acid selected from sulfonicacids, imidic acids, and methide acids, upon exposure to high-energyradiation.
 4. The resist composition of claim 1 wherein the polymerfurther comprises recurring units having the general formula (U-2):

wherein s is 0 or 1, t is an integer of 0 to 2, R¹, R², and B¹ are asdefined above, c is an integer satisfying c≦5+2t−e, d is 0 or 1, e is aninteger of 1 to 3, X is an acid labile group when e is 1 or X ishydrogen or an acid labile group when e is 2 or 3, at least one X beingan acid labile group.
 5. The resist composition of claim 1 wherein thepolymer further comprises recurring units having the general formula(U-3) and/or (U-4):

wherein f is an integer of 0 to 6, R³ is each independently hydrogen, anoptionally halo-substituted C₁-C₆ alkyl or primary or secondary alkoxygroup, or an optionally halo-substituted C₁-C₇ alkylcarbonyloxy group, gis an integer of 0 to 4, and R⁴ is each independently hydrogen, anoptionally halo-substituted C₁-C₆ alkyl or primary or secondary alkoxygroup, or an optionally halo-substituted C₁-C₇ alkylcarbonyloxy group.6. The resist composition of claim 1, further comprising a basiccompound.
 7. The resist composition of claim 1 which is subject to ArF,KrF, EB, EUV or X-ray lithography.
 8. A pattern forming processcomprising the steps of applying the chemically amplified resistcomposition of claim 1 onto a processable substrate to form a resistfilm, exposing patternwise the resist film to high-energy radiation, anddeveloping the exposed resist film in an alkaline developer to form aresist pattern.
 9. The pattern forming process of claim 8 wherein theprocessable substrate has the outermost surface of a chromium-containingmaterial.
 10. The pattern forming process of claim 8 wherein theprocessable substrate is a photomask blank.